Coal can be described as a coal complex polymer or macromolecule consisting of a condensed aromatic carbon-atom lattice surrounded by a typical “fringe” formed by functional side groups. It can also be described as a heterogeneous mixture composed of a macromolecular network with varying degrees of cross-linking. Coal consists of modified lignin, as well as cellulose and melanoidin-type materials which are considered to be the “backbone” of the macromolecular network. Cross-linkage is generally dominated by alkyl and aryl ether groups, especially in low-rank coal, with oxygen functional groups, while the degree of aromaticity tends to increase with coal rank. Because of its complexity and heterogeneity, it is very difficult to depolymerize and solubilize coal without subjecting it to extreme physical (temperature, pressure, etc.) and/or chemical (pH, redox potential, solvation energy, etc.) conditions.
Manganese peroxidase (MnP, Enzyme Commission Number (EC) 1.11.1.7) is one of the most common and efficient extracellular lignin-modifying heme-peroxidases secreted by “classic” white-rot fungi (See, e.g., Hofrichter, M., 2002, Enzyme and Microbial Technology 30, 454-466; Martinez, 2002, Enzyme and Microbial Technology 30, 425-444; Hatakka, A. et al., 2003. Manganese peroxidase and its role in the degradation of wood lignin. In Mansfield SD, Saddler JN (eds) Applications of Enzymes to Lignocellulosics, ACS Symposium Series 855. American Chemical Society, Washington D.C., Chapter 14, 230-243; and Hatakka A. et al., 2010. Fungal biodegradation of lignocelluloses. In: Hofrichter M. (ed.) The Mycota, X, Industrial applications, 2nd Ed. Springer, Berlin Heidelberg N.Y.). The enzyme has been shown to efficiently oxidize a number of recalcitrant polymers (e.g., polycyclic aromatic hydrocarbons, organohalogens, nitroaromatic compounds, and natural substances like lignins, milled wood and humic substances) derived from low rank coal or low-rank coal and other persistent aromatics in cell-free reaction systems (in vitro) (See, e.g., Hofrichter et al., 1996, Appl. Microbiol. Biotechnol. 46, 220-225; Hofrichter et al., 1997 a. Appl. Microbiol. Biotechnol. 47, 419-424, and Hofrichter, 1997 b. Appl. Microbiol. Biotechnol. 47, 566-571; Ziegenhagen et al., 1998, J. Basic Microbiol. 38, 289-299; Hofrichter et al., 1998, Appl. Microbiol. Biotechnol. 49, 584-588; Hofrichter et al., 1999, Appl. Microbiol. and Biotech. 52, 78-84; Hakala et al., 2006, Appl Microbiol. Biotechnol. 73, 839-849; and Hofrichter et al., 2010, Appl. Micribiol. Biotechnol. 87, 871-897.). The fungus, Paecilomyces variotii is known to produce a variety of enzymes including tannase.
Manganese peroxidase belongs to the class II peroxidase group of the plant peroxidase superfamily that is characterized by a protoporphyrin IX (heme) as a prosthetic group in the active center (Welinder, 1992, Current Opinion in Structural Biology 2, 388-393; Poulos et al., 1978, J. Biol. Chem. 253, 3730-3735; Piontek et al., 1993, FEBS Letters 315, 119-124; and Hofrichter et al., 2010, supra.). The catalytic cycle of the enzyme behaves like other well-known heme peroxidases such as lignin peroxidases (LiP, EC 1.11.1.14) and the peroxidase of Coprinopsis cinerea (CiP, EC 1.11.1.7), except that MnP uses Mn2+ ions as the preferred electron donor. The catalytic cycle is activated by H2O2. The native MnP is oxidized to intermediate forms which then oxidize Mn2+ to Mn3+ and return it to its native form. Manganese(III) is highly reactive and both chelated and stabilized by organic acids such as oxalate or malonate (See, e.g., Wariishi et al., 1992, J. Biol. Chem. 267, 23688-23695; and Hofrichter et al., 2001, Appl. and Environ. Microbiol. 67, 4588-4593). Chelated Mn3+ ions act as strong, diffusible redox mediators that are able to attack organic bonds in large biopolymers non-specifically.